Dangling Dns in Buffalo

In Buffalo applications, Dangling DNS typically emerges from two patterns: (1) using dynamic subdomains for multi-tenancy or user-generated content without proper DNS lifecycle management, and (2) misconfigured redirects that trust user-supplied URLs or hostnames. Buffalo's convention-over-configuration approach can inadvertently create these vulnerabilities when developers rely on automatic routing or third-party service integrations without validating DNS ownership.

Attack Pattern 1: Subdomain Takeover via Stale CNAME Records
Buffalo apps often map routes like /users/:username to subdomains (username.yourbuffaloapp.com) using middleware such as github.com/gobuffalo/buffalo/middleware. If the app previously used an external service (e.g., AWS S3 bucket, Heroku app, or Azure storage) for a subdomain and decommissioned it without removing the DNS CNAME record, an attacker can register that same service name and hijack traffic. For example:

// vulnerable Buffalo route with subdomain mapping
var routes = buffalo.Routes{
    buffalo.GET("/users/{username}", UserController.Show, middleware.Subdomain("users")),
}

// If DNS has CNAME: username.yourbuffaloapp.com -> s3-bucket.s3.amazonaws.com
// and the S3 bucket was deleted, an attacker creates a new S3 bucket named "s3-bucket"
// and now receives all traffic for username.yourbuffaloapp.com

Attack Pattern 2: Open Redirects Leading to Dangling Domains
Buffalo's buffalo.Redirect helper or custom redirect logic that uses request parameters without validation can point to attacker-controlled domains. If your Buffalo app historically used a custom domain (e.g., legacy.yourbuffaloapp.com) for redirects and later abandoned it, the DNS record may still exist but point to an unclaimed resource:

// vulnerable controller action
func LoginControllerRedirect(c buffalo.Context) error {
    redirectURL := c.Param("url") // user-controlled
    return c.Redirect(302, redirectURL) // no validation
}

// Attacker crafts: https://yourbuffaloapp.com/login?url=https://legacy.yourbuffaloapp.com/malicious
// If legacy.yourbuffaloapp.com CNAME points to a deleted Heroku app, attacker claims it

Buffalo's pop middleware and asset pipeline (buffalo.AssetServer) can also expose dangling DNS if they reference external CDNs or storage with stale DNS entries. The risk is compounded because Buffalo's default buffalo.New setup includes middleware.CanonicalHost only if explicitly configured, leaving many apps without host validation.

Detecting dangling DNS in Buffalo requires correlating your application's runtime behavior with DNS records. middleBrick automates this by:

• Extracting hostnames from Buffalo routes: Scanning your live API endpoint (e.g., https://api.yourbuffaloapp.com) and analyzing responses for redirects (Location headers), Link headers, and JavaScript-generated URLs that reference subdomains or legacy domains.
• Cross-referencing with OpenAPI specs: If your Buffalo app exposes an OpenAPI/Swagger spec (generated via buffalo generate openapi), middleBrick resolves $ref definitions to find all host, servers, and externalDocs fields that may list domains.
• DNS enumeration: For each discovered hostname, middleBrick queries DNS records (CNAME, A, AAAA) and checks if they point to cloud provider default domains (e.g., .s3.amazonaws.com, .herokuapp.com, .azurewebsites.net) that are unclaimed or return NXDOMAIN.

Example scan output from middleBrick's CLI:

$ middlebrick scan https://yourbuffaloapp.com

[FAIL] Dangling DNS (BOLA/IDOR)
  Severity: High
  Description: CNAME record "legacy.yourbuffaloapp.com" points to "deleted-app.herokuapp.com" (NXDOMAIN).
  Evidence: GET /login?url=https://legacy.yourbuffaloapp.com returns 302 to legacy.yourbuffaloapp.com
  Remediation: Remove DNS CNAME or reclaim the Heroku app; implement host validation in Buffalo middleware.

[INFO] Scanned 12 categories in 8.2s. Score: C (67/100)

The scanner's LLM/AI security checks also probe for dangling DNS in AI endpoints—if your Buffalo app integrates LLMs via routes like /api/chat, middleBrick tests whether those endpoints accept untrusted host parameters that could lead to subdomain takeover of model hosting services.

Remediate dangling DNS in Buffalo by combining DNS hygiene with application-level controls:

1. Validate Host Headers in Buffalo Middleware
Use Buffalo's middleware.CanonicalHost or create a custom middleware to reject requests with unexpected Host headers. This prevents attackers from exploiting stale DNS records that still resolve to your server:

// config/middleware.go
func HostValidator(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        host := c.Request().Host
        allowed := []string{"api.yourbuffaloapp.com", "yourbuffaloapp.com"}
        for _, a := range allowed {
            if host == a {
                return next(c)
            }
        }
        return c.Error(400, errors.New("invalid host"))
    }
}

// In app.go
app.Use(middleware.CanonicalHost("yourbuffaloapp.com"))
// or
app.Use(HostValidator)

2. Sanitize Redirects with Buffalo's URL Parsing
Never use user input directly in c.Redirect. Instead, validate against a whitelist or use Buffalo's urlpkg to parse and check the hostname:

import "net/url"

func SafeRedirect(c buffalo.Context, rawURL string) error {
    u, err := url.Parse(rawURL)
    if err != nil || (u.Host != "" && u.Host != c.Request().Host) {
        return c.Redirect(302, "/")
    }
    return c.Redirect(302, rawURL)
}

3. Audit DNS Records and Cloud Resources
For Buffalo apps using external services (S3, CloudFront, etc.), ensure DNS CNAMEs only point to actively managed resources. Use infrastructure-as-code (Terraform, Pulumi) to track and automate DNS cleanup. Example AWS S3 bucket policy to block dangling DNS exploitation if a bucket is unintentionally public:

{
  "Version": "2012-10-17",
  "Statement": [{
    "Effect": "Deny",
    "Principal": "*",
    "Action": "s3:GetObject",
    "Resource": "arn:aws:s3:::your-buffalo-assets-bucket/*",
    "Condition": {
      "StringNotEquals": {
        "aws:Referer": "https://yourbuffaloapp.com"
      }
    }
  }]
}

4. Monitor with Buffalo's Built-in Tools
Enable Buffalo's pop middleware logging to capture redirects and external calls. Combine with middleBrick's Pro continuous monitoring to get alerts when new dangling DNS entries appear. In your .buffalo.dev.yml, configure scan schedules:

# .buffalo.dev.yml
security:
  scans:
    - schedule: "0 0 * * *" # daily
      threshold: 80 # fail if score drops below B

These steps ensure your Buffalo app doesn't become a vector for subdomain takeover, protecting both your brand and user data.

Q: How does Buffalo's middleware help prevent dangling DNS?
A: Buffalo's middleware.CanonicalHost enforces a single, valid hostname for all requests, blocking traffic to stale DNS records that might resolve to your server. Custom middleware can extend this to validate redirects and subdomain usage.

Q: Can middleBrick detect dangling DNS in Buffalo apps using custom domains?
A: Yes. middleBrick scans your live endpoint, follows redirects, and analyzes DNS records for any hostname referenced in responses—including custom domains configured in Buffalo's buffalo.Config or OpenAPI spec. It flags CNAMEs pointing to unclaimed cloud resources (e.g., .s3.amazonaws.com with no bucket).